Eleven
Thousand
Metres

0 – 200 metres

The Sunlight Zone

Epipelagic

Everything you have ever pictured when you pictured the sea lives in this thin bright film. It is less than two percent of the ocean's volume, and it holds almost all of its photosynthesis. Plants only work where there is light, and the light stops here.

Your dolphins can dive to about 300 metres, but they don't like to. They have lungs. Everything with lungs is a visitor down here — the deep is on loan, and the surface is always the debt.

• 90% of marine life lives in this layer
• 2% of ocean volume
• ~25°C at the equator, falling fast with depth

200 – 1,000 metres

The Twilight Zone

Mesopelagic

Too dark for plants, just light enough for eyes. So evolution built eyes. Enormous, upward-pointing, tubular eyes that hunt for silhouettes against the last of the sky.

And in response, everything else learned to erase its own silhouette. Hatchetfish wear rows of lights along their bellies tuned to exactly match the brightness above them, so that from below they simply are not there. This is called counter-illumination. It is camouflage made of light.

Every night, the animals here rise hundreds of metres to feed in the dark and sink again before dawn. It is the largest migration on Earth, it happens every single day, and it involves more biomass than every land animal combined.

• 1% of surface light reaches 200 m
• 0% reaches 1,000 m
• Nightly vertical migration — the biggest on the planet

1,000 – 4,000 metres

The Midnight Zone

Bathypelagic

No sun has ever reached here. Not dim — absent. The only light in this zone is light that something living decided to make, and roughly four out of five animals down here can make it.

The anglerfish carries a lure of glowing bacteria on a modified fin ray, held out in front of a mouth that can swallow prey larger than itself. The male of some species is a fraction of her size; when he finds a female he bites on, fuses to her skin, and slowly dissolves into a pair of gonads she carries for the rest of her life. Finding anyone at all down here is so unlikely that it was worth giving up being a fish.

Sperm whales hunt through this darkness on a single held breath, shouting at 230 decibels and listening for squid in the returning echo.

• ~4°C — and it stays that way, everywhere, forever
• ~80% of species here are bioluminescent
• 2,000 m — a sperm whale's working depth

4,000 – 6,000 metres

The Abyss

Abyssopelagic

From the Greek ábyssos — bottomless. This is most of the ocean: a cold, black, near-empty plain covering more than half the surface of the planet, under four hundred atmospheres of pressure.

Nothing grows here, so everything here eats what falls. A constant slow drizzle of dead plankton, scales, shells and faeces sinks from the lit world above — marine snow, the same flecks drifting up past you now. It can take weeks to reach the bottom. Whole ecosystems are built on other people's litter.

The dumbo octopus flaps two small ear-like fins and hovers, one of the few things down here in no particular hurry.

• ~400 atm — 400× the pressure on your skin right now
• >50% of Earth's surface is abyssal plain
• Weeks for a flake of marine snow to fall this far

6,000 – 11,000 metres

The Trenches

Hadopelagic — named for Hades

Below six kilometres the seafloor cracks open into trenches. They make up a vanishing fraction of the ocean, and they are the least visited places on the planet. More people have been to the Moon than to the bottom of one.

Life does not stop. The hadal snailfish — pale, soft, no scales, no swim bladder — holds itself together with a molecule called TMAO that stops the pressure from crushing its proteins. Below about 8,200 metres the chemistry stops working, and that appears to be a hard ceiling on fish. Not a coincidence: it's the same number in every trench on Earth.

• 8,336 m — deepest fish ever filmed, Izu-Ogasawara Trench, 2022
• ~46 hadal trenches worldwide
• <30 people have reached full ocean depth